Process for removing and/or preventing the deposition of scale in evaporators



Patented Jan. 25, 1938 PROCESS FOR REMOVING AND/OR PRE- VENTING THEDEPOSITION OF SCALE IN EVAPORATORS Charles Fitch Weston, Prince GeorgeCounty, Va., assignor, by mesne assignments, to The Solvay ProcessCompany, New York, N. Y., a corporation of New York No Drawing.Application November 25, 1932, Serial No. 644,329

14 Claims.

This invention relates to a process for preventing the deposition ofscale and/or for removing scale deposited from heated aqueous liquorscontaining calcium or magnesium and bicarbonate ions, and particularlyto a process for preventing the deposition of scale and/or removingscale deposited from solutions of alkali metal nitrates such as sodiumor potassium nitrate containing calcium or magnesium and bicarbonateions during the evaporation of such solutions.

An alkali metal nitrate may be produced for example, by absorbingnitrogen oxides in a solution of sodium carbonate to give in the laterstages of the absorption a nitrate solution containing free nitric acid.This sodium nitrate solution frequently contains calcium and/ormagnesium ions which may originate either in the soda ash or in thewater used for making up the soda ash solutions in which the nitrogenoxides are absorb-ed. Before evaporation the acid liquor is usually madeslightly alkaline with sodium carbonate to obviate the corrosive actionof an acid liquor on the evaporators. When the alkaline liquor, nowcontaining calcium and/or magnesium ions and bicarbonate ions, isevaporated, calcium and magnesium compounds deposit as scale on theheating surfaces of the evaporator and materially affect the efiiciencyof the evaporation process.

I have found that by preventing the presence of or removing bicarbonateions present in the solutions during their evaporation, the depositionof scale may be prevented and further, that calcium and magnesium scaleswhich may have been previously deposited on the surfaces of theevaporator may be removed, and that these results may be readilyaccomplished by having present in the liquor to be evaporated anammonium salt in sufficient quantity. I have further found that inevaporating solutions of soluble materials to crystallize solid from thesolution undergoing concentration, the deposition of scale may beprevented by having present in the solution an ammonium salt during thefirst stages of the evaporation of the solution and that in a laterstage of the evaporation, particularly in the last stage whereincrystals are formed and a mixture of crystals and mother liquor is beingconcentrated, the liquor may be freed of ammonium salt without a scaledepositing during the final heating and concentration.

It is accordingly an object of this invention to provide a processwhereby the deposition of scale from aqueous liquors being heated may beprevented and particularly for preventing scale deposition from alkalimetal nitrate solutions during concentration which solutions containcalcium and/or magnesium ions and bicarbonate ions. It is a furtherobject of this invention to provide a process whereby calcium and/ormagnesium scale on evaporator surfaces may be readily removed therefrom.

In carrying out this invention a liquor containing calcium or magnesiumand bicarbonate ions is heated with an ammonium salt of an acid formingsalts of calcium and magnesium more soluble than calcium or magnesiumcarbonate, the ammonium salt being present in amount in excess of thatequivalent to thecarbonate which is present in the solution, both asnormal carbonate and as bicarbonate. By heating the solution containingan ammonium salt such as, for example, ammonium nitrate, the solutionmay be concentrated in contact with heat transfer surfaces without ascale containing calcium or magnesium depositing upon the surfaces ofthe evaporator and, if such a scale deposit is already present upon theevaporator surfaces, it is removed in the course of evaporating thesolution containing an excess of the ammonium salt. 'l

Further, in carrying out this invention, a solution of a solublematerial containing calcium or magnesium and bicarbonate ions is heatedwith an ammonium salt as described above, thereby preventing thedeposition of scale during the concentration of the liquor. Theconcentrated liquor is then treated to decompose the ammonium saltpresent, for example by heating with an addition of a soluble basicmaterial such as sodium or potassium hydroxide or carbonate in amountsuflicient to decompose the ammonium salt in the concentrated liquor,and this heating of the liquor is preferably carried out with solidparticles of the dissolved material, which have crystallized fromsolution, in suspension in the liquor.

It is believed that the process of this invention depends for itseffectiveness in preventing the deposition of or removing scale duringthe evaporation of a solution, upon the removal from the solution ofbicarbonate and carbonate ions in accordance with the equations heat 1N112; co; Nm 11003 I 1 heat n NHI 1100; N113 C02 1120 It is furtherbelieved that the prevention of scale deposition from the liquor in thefinal stages of evaporation, and particularly where a mixture ofcrystals and mother liquor is being concentrated, with no ammonium saltpresent and with calcium and/0r magnesium ions present, is a result ofone or both of the following factors: the solid crystals in suspensionin mother, liquor may have a scouring action on the evaporator surfacesas the mixture of mother liquor and crystals is agitated during theheating which prevents insoluble calcimn and/or magnesium compoundswhich .are. thrown out of solution from depositing as a scale upontheiheating surfaces and/ or, the addition of sodium carbonate, forexample, to the liquor containing calcium and/or magnesium salts mayform a precipitate of calcium or magnesium carbonate, but suchprecipitate forms in the body of liquor and under such circumstancesdoes not form a scale on the evaporator surfaces. In offering theforegoing theories of the actions by which the deposition of scale isprevented in carrying out my process, it is not intended that myinvention should be limited thereto, since the reasons for no scaleforming in my process are not known with certainty.

The following example is illustrative of a method for carrying out theprocess of this invention: A sodium nitrate solution containing freenitric acid and calcium and/or magnesium compounds, is treated withammonia which, by reaction with the nitric acid contained in thesolution, introduces into the solution a proportion of ammonium nitrate.To the thus treated solution sodium'carbonate is added in amountsuflicient to render the liquor slightly alkaline and the liquor nowcontaining bicarbonate ions is concentrated in a three-effectmultiple-stage evaporator in which the solution is passed throughexternally heated coils or nests of tubes. The proportions of ammoniaand sodium carbonate employed in the preparation of the liquor forevaporation are regulated so that the ammonium nitrate is in excess ofthe amount corresponding to the carbonate as represented by theequationsin which M represents a metal or metal radical. For example,the solution before evaporation may contain about 0.5 gram per liter ofcarbonates calculated as CO2 and 2 grams per liter of ammonium nitrate.The solution may then be evaporated, during which the ammonium nitrateacts to prevent deposition of scale on the heat transfer surfaces of theevaporator or, if calcium or magnesium scale has already been formedupon the evaporator surfaces, this scaly deposit will be removed. Duringthe final step of concentration (for example, in the third stage of athreestage evaporator), in which crystals of sodium nitrate separate outfrom the solution and the material undergoing concentration is a mixtureof solid crystalline sodium nitrate and mother liquor, an alkalinecompound such as sodium hydroxide or sodium carbonate is added to thesolution in amount sufficient to react with the excess ammonium nitratepresent to convert it into sodium nitrate in accordance with thereaction During the final concentration of the liquor the ammonia andcarbon dioxide are evolved from the solution and a liquor obtained whichis free from ammonium salts, and from which an ammonia-free sodiumnitrate may be recovered by concentration and crystallization, without ascale depositing in the tubes of the third stage of the evaporator. Ifdesired, a slight excess of the sodium carbonate may be added over thatrequired for reaction with the ammonium nitrate so that the liquor afterevaporation contains a small amount of sodium carbonate. The liquor thusconcentrated may be cooled to crystallize out additional sodium nitrate,or Without cooling the mother liquor may be separated from the sodiumnitrate crystals and returned, together with fresh sodium nitrate liquorand with the introduction of the desired proportion of ammonium salt,for evaporation and recovery of additional sodium nitrate.

Instead of introducing the ammonium nitrate into the sodium nitratesolution by passing ammonia into an acidic solution, this may likewisebe accomplished by dissolving ammonium nitrate itself in the liquor. Notonly ammonium nitrate but other ammonium salts such as ammonium chlorideor sulfate, the corresponding calcium and magnesium salt of the acidradical of which are more soluble than calcium and magnesium carbonates,may be utilized in accordance with the process of this invention.

I claim:

1. In a process for evaporating a solution of an alkali metal nitratecontaining calcium or magnesium and bicarbonate ions, that improvementwhich comprises introducing into the solution to be evaporated anammonium salt of an acid, the calcium and magnesium salts of which aremore soluble than calcium and magnesium carbonate, said salt being inamount in excess of that equivalent to the carbonate present in thesolution and heating the liquor to evaporate it in contact with heattransfer surfaces.

2. In a process for evaporating solutions of sodium nitrate containingcalcium or magnesium and bicarbonate ions, that improvement whichcomprises introducing into the solution to be evaporated an ammoniumsalt of an acid, the calcium and magnesium salts of which are moresoluble than calcium and magnesium carbonate, said salt being in amountin excess of that equivalent to the carbonate present and heating thesolution to evaporate it in contact with heat transfer surfaces.

3. In a process for the recovery of sodium nitrate wherein a solution ofsodium nitrate containing calcium or magnesium and bicarbonate ions isevaporaated, that improvement which comprises introducing into thesolution to be evaporated an ammonium salt of an acid the calcium andmagnesium salts of which are more soluble than calcium and magnesiumcarbonate, said salt being in excess of that equivalent to the carbonatepresent, evaporating the solution in contact with heat transfer surfacesand then introducing into the evaporated solution a sodium base inamount at least sufficient to react with the residual ammonium saltpresent and heating the thus treated solution until residual ammoniumsalt is substantially decomposed.

4. In a process for the recovery of sodium nitrate from a solutionthereof containing calcium or magnesium and bicarbonate ions, thatimprovement which comprises introducing into the solution ammoniumnitrate in excess of that equivalent to the carbonate present,evaporating the solution in contact with heat transfer surfaces, andthen introducing into the evaporated solution sodium carbonate in amountsuflicient to react with the residual ammonium nitrate present in thesolution and heating the thus treated solution to decompose the ammoniumsalt until the solution is substantially free from ammonium salt.

5. In a process for evaporating solutions of alkali metal nitratescontaining calcium or magnesium and bicarbonate ions to crystallizesolid alkali metal nitrate from solution that improvement whichcomprises introducing into the solution to be evaporated an ammoniumsalt of an acid, the calcium and magnesium salts of which are moresoluble than calcium and magnesium carbonate, said salt being in amountin excess of that equivalent to the carbonate present in the solution,heating the liquor in contact with heat transfer surfaces to evaporateit and crystallize solid alkali metal nitrate from solution, and in astage of said heating wherein the liquor contains solid crystallizedalkali metal nitrate heating the mixture of solid and mother liquor inthe presence of a basic compound of said alkali metal until the residualammonium salt in the mixture is substantially and completely decomposed.

6. In a process for evaporating solutions of sodium nitrate containingcalcium or magnesium and bicarbonate ions by heating the solution instages wherein the solution is passed through externally heated tubesand is concentrated to crystallize solid sodium nitrate duringevaporation that improvement which comprises introducing into thesolution to be evaporated ammonium nitrate in excess of that equivalentto the carbonate present, evaporating the solution containing saidammonium nitrate and in a stage of the evaporation where a mixture ofmother liquor and crystallized sodium nitrate is being heated adding analkaline sodium compound to the liquor in amount sufficient to reactwith the ammonium nitrate present to decompose substantially completelysaid ammonium nitrate.

'7. In a process for the evaporation of a. solution of an alkali metalnitrate containing calcium or magnesium and bicarbonate ions in contactwith heat transfer surfaces, the method of preventing the deposition ofscale on said heat transfer surfaces which comprises introducing intosaid solution an ammonium salt of an acid the calcium and magnesiumsalts of which are more soluble than calcium and magnesium carbonate,said ammonium salt being in excess of that equivalent to the carbonatepresent, evaporating the solution in contact with heat transfer surfacesuntil solid alkali metal nitrate crystallizes therefrom, thenintroducing alkali metal carbonate into the resulting mixture of solidand mother liquor in amount at least sufiicient to react with theresidual ammonium salt present, and heating the thus treated mixture incontact with heat transfer surfaces until said residual ammonium salt inthe mixture is substantially completely decomposed.

8. The process for removing calcium or magnesium carbonate scale fromheat exchange sur faces which comprises heating a solution of an alkalimetal nitrate containing an ammonium salt of an acid the calcium andmagnesium salts of which are more soluble than calcium and magnesiumcarbonate while maintaining the heated solution in contact with thescale on said surfaces, said ammonium salt being present in the solutionin amount in excess of that equivalent to carbonate dissolved in thesolution during the contact of the solution with said scale.

9. The process for the production of an ammonia-free alkali metalnitrate by evaporation to crystallization of a solution of alkali metalnitrate containing calcium or magnesium ions in contact with heatexchange surfaces and simultaneously preventing the deposition of scaleon said surfaces which comprises introducing into said solution anammonium salt of an acid the calcium and magnesium salts of which aremore soluble than calcium and magnesium carbonate, adding to thesolution alkali metal carbonate in amount sufficient to render thesolution alkaline, the amounts of said ammonium salt and carbonate beingsuch that the solution contains an excess of the ammonium salt, heatingthe solution in contact with said heat exchange surfaces to concentrateit, and then adding to the thus concentrated solution a basic compoundof an alkali metal and heating the solution to decompose substantiallycompletely the residual ammonium salt contained therein.

10. In a process for evaporating a solution of an alkali metal nitratecontaining calcium or magnesium and bicarbonate ions, that improvementwhich comprises introducing into the solution to be evaporated ammoniumnitrate in amount in excess of that equivalent to the carbonate presentin the solution and heating the liquor to evaporate it in contact withheat transfer surfaces.

11. In a process for evaporating solutions of sodium nitrate containingcalcium or magnesium and bicarbonate ions, that improvement whichcomprises introducing into the solution to be evaporated ammoniumnitrate in amount in excess of that equivalent to the carbonate presentand heating the solution to evaporate it in contact with heat transfersurfaces.

12. In a process for the recovery of sodium nitrate wherein a solutionof sodium nitrate containing calcium or magnesium and bicarbonate ionsis evaporated, that improvement which comprises introducing intothesolution to be evaporated ammonium nitrate in amount in excess of thatequivalent to the carbonate present, evaporating the solution in contactwith heat transfer surfaces and then introducing into the evaporatedsolution a sodium base in amount at least sufficient to react with theresidual ammonium salt present and heating the thus treated solutionuntil residual ammonium salt in the solution is substantially completelydecomposed.

13. In a process for evaporating solutions of alkali metal nitratescontaining calcium or magnesium and bicarbonate ions to crystallizesolid alkali metal nitrate from solution that improvement whichcomprises introducing into the solution to be evaporated ammoniumnitrate in amount in excess of that equivalent to the carbonate presentin the solution, heating the liquor in contact with heat transfersurfaces to evaporate it and crystallize solid alkali metal nitrate fromsolution, and in a stage of said heating wherein the liquor containssolid crystallized alkali metal nitrate, heating the mixture of solidand mother liquor in the presence of a basic compound of said alkalimetal until the residual ammonium salt in the mixture is substantiallycompletely decomposed.

14. The process for removing calcium or magnesium carbonate scale fromheat exchange surfaces which comprises heating a solution of an alkalimetal nitrate containing ammonium nitrate while maintaining the heatedsolution in contact with the scale on said surfaces, said ammoniumnitrate being present in the solution in amount in excess of thatequivalent to carbonate dissolved in the solution during the contact ofthe solution with said scale.

